I also prefer a narrow five-string. Both of mine are the Alembic 'Classic' taper, 2 nut by 2.5 at the last fret, so the strings almost feel parallel to me. Yamaha later went crazy with those crazy-wide fives on TRB's.
If I had mad money to build a couple of 'what if . . . ?' axes, one of the first things I'd try would be like the fingerboard on a nylon string classical: Utterly straight end-to-end, no taper at all, and a flat-radius (i.e., NO radius) fingerboard, backed with a flat-back neck profile, REALLY a 'U-shaped' neck profile.
It's interesting to me that I was never at home, a four-string was always too much work, the notes just didn't line up right to me. I'd read about Jimmy and Alembic, and a bass with FIVE strings just said to me I'm waiting,Joey, and then when I got my first, the little light went on and everything made sense. There's a part of me that always wants a little P/J Precision (the current candidate is the ESP Frank Bello), but every time I pick up a nice one, something is STILL missing, so it just remains a daydream . . .
Joey